This invention relates to thermal spray techniques for applying high performance, long lifetime coatings. More specifically it relates to a new method and apparatus for efficiently heating and accelerating powder particles to velocities much higher than current practice allows. The increased powder impact velocity on sprayed substrates will provide higher quality coatings than achievable by current practice. This invention further relates to the use of confined capillary arc discharges to produce high pressure and temperature plasma jets for heating and accelerating powder particles to high velocity.
In order to remain competitive in today's industrial market, manufacturers must provide goods with high performance, low cost, and long lifetimes. Thermal spray is a value-added process whereby protective or performance enhancing coatings are sprayed onto components to improve their operating characteristics and to extend component life. Coatings are a pervasive technology, permeating throughout all of industry and high technology applications. Coating technology is an enhancing technology that improves products and reduces cost. In many applications, coatings make it possible to achieve ends that cannot be achieved in any other known way, or in any way that is affordable.
Thermal spraying, in general, is a process for applying coatings of high performance materials, such as metals, alloys, polymers, ceramics, cermets and carbides, onto more easily worked and cheaper base materials. Because of its ability to deposit virtually any material and any combination of materials, thermal spray has a wide and growing range of applications.
Coatings are most easily grouped according to their primary function, although a given coating may provide more than one basic function. The most generally important functional applications are for thermal insulation, wear resistance, corrosion and chemical resistance, and for providing abradable and abrasive coatings, electrically conductive or resistive coatings, medically compatible coatings, dimensionally restorative coatings and polymer coatings, RFI/EMI shielding, and cosmetic repair.
As examples, coatings are applied to increase surface hardness and thus improve wear resistance, to form a protective thermal barrier which allows higher temperature operation, or to improve corrosion or chemical resistance. Coatings are also used to restore and repair the worn surfaces of expensive components, thereby eliminating the cost of replacement. New applications are being discovered daily as the industrial community becomes more familiar with thermal spray coatings and as available coating quality improves.
Spray materials are typically supplied as powders with typical particle sizes ranging from 5 .mu.m to 100 .mu.m or more in size. In recent years, powders less than 5 .mu.m in size are finding some attractiveness. Powders are typically heated to a molten or near molten state by a flame or arc, simultaneously accelerated to a high velocity, and then directed against a substrate. On impact with the substrate, splats are formed, which in turn build up layers of coating material as additional splats impact. A complete coating is produced by moving the spray over the surface to be sprayed, or by moving the part relative to a fixed spray head.
The quality and durability of coatings produced using existing thermal spray techniques could be improved by, among other things, increasing the velocity with which coating particles impact the coated surface and by tighter control of the chemical and thermal environment experienced by the particles during flight. Needs exist for thermal spray techniques that increase the particle velocity and provide for better control of the chemical and thermal environment seen by the particles during flight.
Potential applications for thermal spray techniques are far reaching and include, but are not limited to, thermal barrier coatings in gas turbines, protective coatings for rocket nozzles and internal combustion engine components such as piston rings, prosthetic device coatings, coatings for hardfacing load bearing surfaces, anti-corrosion coatings for bridges and other infrastructure, and specialized coatings for the surfaces of rails and insulators in electromagnetic guns. Fabrication of ceramic substrates for electronic circuits by thermal spray techniques has long been considered, but is not currently widely accepted. That situation could be expected to improve with the availability of improved spray processes which provide denser coatings with more uniform, more homogenous and more resistive (i.e. less conductive) coatings. Needs exist for thermal spray apparatus and methods that provide dense, hard and uniform coatings with strong resistive properties.
A specific need exists to improve the lifetime and performance of military gun system components. Currently, all gun systems are limited in their performance and lifetime due to exposure to high temperature gases and, in the case of electromagnetic launchers, due to high current arc discharges. The bore components of advanced electromagnetic rail guns experience very severe and hostile environments which greatly limit their projected lifetimes. Rocket and missile performance could also be dramatically improved by increasing rocket chamber and nozzle operating temperatures. Materials exist that may dramatically improve the performance and lifetime of launchers, but in most cases those materials are either very expensive or otherwise inappropriate for fabrication of entire parts. Research has shown that specialized coatings reduce the erosion of gun components and that tungsten base alloys, for example, show essentially no erosion for heat fluxes and exposure times of useful interest. Specifically, the performance of new hypervelocity guns such as the Electric Light Gas Gun (ELGG), the Combustion Light Gas Gun (CLGG), electrothermal-chemical guns and electromagnetic guns would see dramatic performance improvements if barrels and chambers were coated with materials that provided safe operation at high temperatures for long lifetimes. Needs exist for coating methods and apparatus which effectively use small amounts of those materials as specialized coatings for reducing system cost and for increasing performance in guns and missiles.
The performance of internal combustion engines, gas turbines, steam turbines and many other devices could also be dramatically improved if higher operating temperatures could be tolerated along with increased erosion and corrosion resistance. Advanced thermal barrier coatings could provide this improvement.
Dimensional restoration of aircraft worn landing gear assemblies could be a very large potential market for advanced thermal spray technology, but the quality of currently available thermal spray coatings is not sufficient for the FAA to approve the process for restoration of these and other critical aircraft assemblies. The potential to achieve true metallurgical bonding at high velocity holds the promise of acceptance of thermal spray repairs for these assemblies, opening up a large new market that does not now exist. Restoration of landing gear is a strong contender for an early niche market for pulsed electrothermal powder spray, since it is a very high value added service that cannot be achieved with any other known thermal spray process. The savings to the airline industry are expected to be in the millions of dollars annually.
Metallurgical bonding is also a desirable feature for any metallic coating, especially those used for heavy equipment which often requires repair or replacement of heavy load bearing surfaces. WC-Co is one such coating material used in such application. If such coatings could be sprayed on with a smoother as sprayed surface, lower cost would result since post processing of the surface would not be required. Needs exist for a thermal spray apparatus which can achieve true metallurgical bonds between the sprayed coating and the material substrate.
Thermal spray includes a variety of approaches, but can be grouped into three main coating processes: combustion, wire-arc, and plasma. Each approach has its advantages and disadvantages that tend to position it in particular areas of application. Those approaches include (in roughly ascending order of coating quality and with particle impact velocities listed in parentheses): flame spray (30 m/s), flame wire spray (180 m/s), wire-arc spray (240 m/s), conventional plasma spray (240 m/s), detonation gun (910-1200 m/s), high efficiency oxyfuel (610-1200 m/s), high-energy plasma (240-1220 m/s) and vacuum plasma (240-610 m/s). Thermal spray techniques are further subdivided into continuous and detonation processes.
Most of the approaches listed above involve continuous processes. The detonation gun is the most notable exception. While the detonation gun produces some of the highest quality coatings, the pressures and temperatures attainable are limited due to the combustion process used, and only incremental improvements can be expected. Recent improvements in high-velocity oxyfuel spray makes it competitive with detonation gun-applied coatings in some applications. High-velocity oxyfuel spray coating quality is roughly comparable to coatings applied by detonation guns.
Thermal spray has a rich history, but there is considerable room for improvement in the technology. As related by Thorpe and by Berndt, et al, there are substantial limitations in existing thermal spray apparatus and methods which have slowed or prevented the expansion of existing markets and the penetration of new markets and new application areas. The quality of coatings produced by existing thermal spray technology and the economic viability of the coatings produced are limited by numerous factors including:
a need for higher particle impact velocities, which generally produce better coatings; PA1 a need for more uniform spray patterns, with more uniform spatial and temporal velocities desired; PA1 lack of sensitivity to feedstock materials and other process variables; PA1 inefficient use of the energy used to melt coating materials; PA1 deposition inefficiencies, which are less than 50% for some materials; PA1 coating properties that are not equivalent to those of wrought material; PA1 high cost of high-performance materials; PA1 inadequate coating consistency and reproducibility; PA1 unreliable equipment; PA1 low spray rates; and PA1 lack of industry standards for spray guns and few for coating materials. (No coating may be considered generic and reproducible at this time, partly due to the inability to accurately model existing systems.) PA1 uniform and controllable velocities of particles on impact; PA1 sufficient velocity to produce a high density deposit without "exploding" the molten or partially molten droplets on impact; PA1 uniform and controllable heating of particles; PA1 attainment of fully molten or plastic particles without vaporization or undesired reactions; PA1 isolation from or controlled interaction with the ambient environment; and PA1 stable process conditions with highly reproducible results. PA1 Higher velocity impact (2000-4000 m/s, which is 2-3 times conventional technology) PA1 Can melt anything PA1 Independent control of thermal and chemical environment, controllable working gas PA1 Potential for true metallurgical bonding rather than just mechanical gripping of the surface PA1 No combustible gases used making system much safer PA1 No vacuum system needed, high purity maintained by use of inert working gases PA1 Working fluid can be tailored for specific powders independent of energy input PA1 Potential elimination of grit blasting prior to spraying due to higher velocity impact PA1 Can achieve more uniform spray pattern and particle velocity distribution PA1 Higher performance will allow use of cheaper coating materials PA1 Very high velocity allows high quality coatings at greater spray angles of incidence PA1 Reduced substrate heating PA1 Advanced operating modes include functionally gradient coatings using multiple powder feed ports for multiple powder types fed in alternating sequences from one pulse to the next PA1 Hardware readily scaled to large or small sizes, power levels, and deposition rates PA1 Can readily model gun performance, reducing testing times
Needs exist for thermal spray technology that addresses these limitations.
Particle impact velocity is one of the most important factors in coating quality. One of the main areas of research and innovation in the industry has been the quest for ever higher velocities. Higher velocity impact generally produces denser, harder and more uniform coatings having lower porosity and higher adhesion and cohesion. In addition, higher velocity impact tends to produce coatings with less induced stresses.
Acceleration of a single coating particle is determined by solving the drag equation [ref] ##EQU1## where v.sub.p, .rho..sub.p, and d are the particle velocity, density and diameter respectively, .rho..sub.g and v.sub.g are the gas density and velocity as determined by the fluid equations, and C.sub.d is the drag coefficient, which is approximately 0.44 for most cases of interest in thermal spray. It will be clear to one skilled in the art of fluid flow that this equation tells us that for a given particle size and density, the determining factors are the velocity of the gas relative to the particle and the density of that gas. The higher the gas density and the higher the relative velocity, the stronger is the accelerating force on the particle. In essence, one of the goals of all thermal spray devices is to maximize this quantity.
Virtually all thermal spray devices currently operate at or near atmospheric gas density, and thus attempts to increase powder velocity focus mostly on increasing the gas velocity. However, increases in gas velocity are quite limited due to the combustion processes used in flame spray, HVOF, detonation gun and the like, with only incremental improvements being made over periods of many years.
As related by Berndt, et al, needs exist for coating processes and apparatus having the following characteristics:
Despite the limitations of existing thermal spray systems, a large market has developed over the years. However, penetration of thermal spray technology into new application areas, such as the automotive industry and electronics, is fundamentally constrained by limitations in the existing technology and by high cost. Needs exist for economically viable thermal spray technology that addresses those limitations and that provides for the expansion of existing applications and the entry of the technology into new markets.
High quality coatings are difficult to make due to the high temperatures required to melt materials and due to the difficulty of accelerating powders to high velocity. Factors contributing to those difficulties include the lack of control over the chemical environment and the inability to prevent oxidation reactions from occurring on the surfaces of the powder particles prior to impact on the substrate. High quality coatings are also difficult to make due to the requirement for high impact velocity of the powders on the substrate. It is difficult to achieve velocities above the state of the art as described above.
Needs exist for thermal spray apparatus and methods that control the chemical and thermal environments of the particles and increase the impact velocity significantly above about 1200 m/s. Velocities above 2000 m/s are of great interest to researchers and developers of new coatings
A good coating also requires the proper thermal state of the particle, typically either molten or in a plastic state just below the melting point. In many cases such as for WC-Co, it is desirable to have the powder particle in a plastic state rather than completely molten. The particle must be exposed to sufficiently high temperature gas to achieve this state. In addition, the density of that gas must be sufficiently high so that the enthalpy of the gas is high enough to provide sufficient energy to bring the particle to that state on a fast enough time scale and without significant cooling of the plasma.
Researchers have investigated electrical means of increasing the gas flow velocity as a means of increasing powder impact velocities. In U.S. Pat. No. 3,212,914 the use of pulsed electrical discharges for accelerating powders was investigated, but did not confine the arc and utilized low gas densities, thus limiting the effectiveness of the technique. In U.S. Pat. No. 4,142,089 the use of coaxial railguns to spray powders is described. This is a form of electromagnetic accelerator, using magnetic forces on an arc discharge to snowplow and accelerate the gas in front of the fast moving arc. The arc then accelerates powders placed in its path. Such devices have not been commercialized and would have severe practical problems due to arc damage to the metallic cylindrical rails, the need for high current and high current switches, and due to the low gas density required for coaxial gun operation.
In a separate investigation into the use of electrothermal discharges for accelerating powders, Shcolnikov, et al have reported on the use of a plasma jet utilizing an ablative capillary insulating liner. They achieved potentially interesting velocities. However, they give no instruction as to how to convert such a device into a practical repetitively operating commercial device. The ablative liners only last for a very small number of pulses before the ablative liner material must be mechanically replaced, a very time consuming and expensive process not amenable to commercial operation. They used too small a capillary, too high a temperature, and too short a pulse length to achieve desirable commercial performance. They recognize the need to develop a method of plasma replenishment but give no instruction as to how to do this.
Historically, plasma jets have been practiced almost exclusively using ablative plastic liners. The main exception is the Pulsed Electrothermal Thruster described in U.S. Pat. Nos. 5,425,231 and 4,821,509 and 4,821,508 in which a repetitively operated capillary discharge is continuously fed by a low molecular weight gas feed and produces pulses of exhaust at 100's to 1000's of pulses per second for the purpose of producing thrust for rocket propulsion in vacuum.
Needs exist for a method which can make use of the power and control possible with confined capillary arc discharges. Needs exist to identify a repetitively operating hardware concept which maximizes the quantity .rho..sub.gas (v.sub.gas -v.sub.powder).sup.2, provides sufficient thermal energy and heating time, and whose critical hardware components would survive for commercially reasonable times. The main issue for ceramic walled capillary discharges is how to introduce a working fluid into the capillary region in a repetitive manner which not only allows the capillary liner to survive essentially indefinitely, but also provides sufficient mass to accelerate the powder without exceeding chamber and barrel temperature limits. Calculations show that it is desirable to operate the capillary discharge at roughly 1000 atmospheres peak pressure at a peak temperature of about 12,000K to achieve the desired performance.